One prior art digital valve comprises the following elements which it shares in common, at least in part, with a valve in accordance with the invention:
a digital valve comprising:
an inlet duct disposed on a valve axis in order to receive a fluid at an upstream pressure via an upstream side, and an outlet duct disposed on the same axis in order to allow said fluid to leave via a downstream side under a downstream pressure which is substantially smaller;
a plurality of shutters, each serving, when open, to disengage an orifice allowing a flow of said fluid to pass from said inlet duct to said outlet duct at a rate which is individual to said shutter, and serving, when closed, to prevent said flow;
a valve body forming a plurality of seatings angularly distributed around said valve axis around a ring of shutters with the diameter of the ring being greater than the diameter of said inlet duct, at least, said seatings each receiving a corresponding one of said shutters in such a manner as to connect said shutters in parallel between said inlet duct and said outlet duct so that the flows through open shutters are added together; and
a plurality of actuators each for actuating a corresponding one of said shutters.
In this prior art valve, shutters having different specific flow rates are fitted in identical seatings, thereby enabling a common valve body to be used for different sets of shutters, at least within limits. The shutters are angularly distributed around the axis of the valve since that makes it relatively simple firstly to receive the shutters and their actuators, and secondly to obtain independent access to each of the shutters for repair or replacement, if necessary. The coaxial disposition of the inlet and outlet ducts facilitates inserting the valve in a rectilinear line of pipework.
One such prior art digital valve is described, for example, in U.S. Pat. No. 3,937,248.
In general, the specific flow rates of the various shutters are distributed, with reference to a unit flow rate which is the smallest possible flow rate, over successive terms of a geometrical progression having a ratio of 2, i.e., for example, 1, 2, 4, 8, 16, 32, etc. By adding such terms together it is possible to obtain any integer number in the range 1 to a maximum number, e.g. 63, using a minimum number of terms. In other words, such a distribution of shutter flow rates has the advantage that every intermediate flow rate can be obtained to within one flow rate unit using a minimum number of shutters. However, other distributions are possible and may be more advantageous.
An essential quality of such valves may be called the exactness with which they add flow rates. When this quality is present, the flow rate of the valve is always equal to the sum of the individual flow rates of the open shutters regardless of which shutters are open and regardless of how many shutters are open, with the individual flow rate of each shutter being the rate that flows therethrough when it is the only one of the shutters to be open. This quality of exactness is lost when the flows passing through two open shutters interact with each other in such a manner as to cause the fluid pressure and/or speed to be modified in the vicinity of either shutter, in which case the flow rate through the shutter is different from its individual flow rate. This quality of exactness is easier to obtain if the pressure difference between the upstream and the downstream sides of the valve is increased, i.e. if the head loss through the valve is increased and/or if the size of the valve is increased relative to the flow rates to be controlled, i.e. if its bulk is increased.
Prior art digital valves suffer from drawbacks with respect to one or other of the following points:
valve manufacturing cost;
cost of adapting a valve to a different flow rate step size;
ease with which various shutter modules can be removed for repair or replacement;
bulk;
exactness of flow rate addition;
head loss;
speed of changeover between two successively controlled flow rate values; and
transmitting large flow rates.
The object of the present invention is to remedy such drawbacks of prior art digital valves in simple manner.
More particularly, the present invention seeks to reconcile great ease in removing each of the shutter modules with a valve which is compact and which has accurate flow rate addition.